An interior permanent magnet synchronous motor (IPM) is a motor having a layer of permanent magnet placed inside a rotor and primarily utilizing permanent magnet torque and utilizing auxiliary reluctance torque.
Resultant formula of the reluctance torque and the permanent magnet torque is as follows: T=mp(Lq−Ld)idiq+mpψPMiq.
In the above formula, T is an output torque of a motor, the performance of the motor can be improved by increasing the value of T; the first item in the equation following T is the reluctance torque, and the second item is the permanent magnet torque; ΨPM is the maximum value of stator-rotor coupling magnetic flux generated by a permanent magnet of the motor, m is a phase number of a conductor of a stator, Ld and Lq are inductances along axis d and axis q respectively, wherein axis d refers to an axis coincided with an axis of the main magnetic pole, and axis q refers to an axis perpendicular to the axis of the main magnetic pole, the perpendicular relationship refers to perpendicularity of electrical angles, and id and iq are components of an armature current in the directions of axis d and axis q respectively. As can be seen from the above resultant formula, the output torque of the motor T can be increased by increasing both the permanent magnet torque as the second item and a difference of the inductances along axis d and axis q of the motor.
In the prior art, the performance of the motor is generally improved by improving the performance of the permanent magnet, that is, by increasing the permanent magnet torque to increase the value of the resultant torque so as to improve the efficiency of the motor, and the common method is to use rare-earth permanent magnets. However, since rare earth is a non-renewable resource and is expensive, the widespread use of this kind of motor is restricted. In addition, due to the limited volume of the rotor, the occupation ratio of the permanent magnets in each pole of the rotor has a limit value, thus it is difficult to increase the output torque of the motor by increasing the usage amount of the permanent magnetic, which also limits the improvement of the motor efficiency.